Printing techniques broadly fall into two categories. The first is traditional ‘analogue’ techniques such as offset, gravure, flexography or screen printing. In these techniques, a permanent impression of the image to be printed is formed on a printing plate, drum or screen by mechanical or lithographic means, to which ink is applied. The image is then transferred to the material to be printed. In colour printing, this process is repeated a number of times using a different plate, drum or screen and a different ink colour to lay down each colour separation of the image;
The second printing technique is ‘digital’ printing, whereby an image stored in computer memory is formed directly on the material to be printed by an electronically controlled system, without the need for permanent ‘tooling’. Examples of digital print processes are dry powder electrophotography, liquid toner electrophotography, continuous ink jet (CU), thermal ink jet (TIM and piezoelectric drop-on-demand (DOD) ink jet.
In all cases, the printing of colour images involves the sequential application of at least two colour separations.
The printing of packaging materials is a significant part of the global printing market, and the vast majority of this makes use of traditional analogue techniques. However, there is a strong desire within the industry for the benefits offered by digital printing, such as economic short runs, fast response to market demands, affordable personalisation and customisation, reduced wastage and reduced inventory.
Many packaging materials are non-absorbent, typically having an outer surface of metal, plastic or varnish. In order to print onto these materials, existing print processes need to dry or fix each colour separation onto the substrate before putting down the subsequent colour separation, to avoid smudging and degradation of the image; in the case of traditional analogue methods, this would result from the physical contact of the equipment printing the subsequent colours, while in the case of non-contact methods like inkjet, the degradation arises from flow of ink on the non-absorbent substrate.
The need to dry or fix each colour separation before putting down the subsequent colour, adds complexity to the overall printing system. It can also lead to the final result falling outside the customer requirements, either because the fixing method is unable to give the required levels of adhesion (as is the case with thermally cured electrophotographic imaging) or because the chemicals used for fixing are not compatible with the food safety requirements of the end customers (as is the case with UV cured ink jet).
CIJ and TIJ printers use solvent or water-based inks. These processes are not able to create high quality images on non-absorbing substrates because the ink is not viscous, and is able to flow the substrate. Further, the coloured inks used in these small nozzlejet printers are based on soluble dyes, as insoluble pigment based inks can cause problems with nozzle blocking. Therefore the advantages of pigment formulations, namely better water, heat and light fastness, are precluded.
WO93/11866 describes an ink jet printing technology, in which droplets of variable size, containing a high concentration of particulate material, are produced. Specific advantages conveyed by this process include the ability to form droplets smaller than a picoliter while still using pigments as the colorant material. As the size of the droplets is controlled primarily by the voltage waveform applied to an ejection point, they are not limited by the size of an ink jet nozzle. Also, the colorant material is significantly concentrated in the ejected droplets. Therefore, high resolution and high density images based on light and water-resistant pigments can be produced.
WO95/01404 describes ink jet inks suitable for use in the process described in WO93/11866. All the ink compositions disclosed therein contain a binder, such as a synthetic resin.
U.S. Pat. No. 4,165,399 describes binder-less ink compositions suitable for use in ink jet operations. However, these ink composition are only suitable for use on synthetic polymeric resin surfaces (into which the ink composition can penetrate). They are not suitable for use on non-absorbent surfaces. All the ink compositions disclosed therein contain between 0.5 and 5.0 percent by weight of a dye.